E1 INFLUENCE OF TEMPERATURE AND STRAIN RATE ON COHESIVE PROPERTIES OF A STRUCTURAL EPOXY ADHESIVE Thomas Carlberger 1 and Anders Biel 2 1 SAAB Automobile AB, SE-461 80 Trollhättan, SWEDEN 2 University of Skövde, PO Box 408, SE-541 28 Skövde, SWEDEN Abstract Effects of temperature and strain rate on the cohesive relation for an engineering epoxy adhesive are studied experimentally. Two parameters of the cohesive laws are given special attention: the fracture energy and the peak stress. Temperature experiments are performed in peel mode using the double cantilever beam specimen. The temperature varies from -40°C to +80ºC. The temperature experiments show monotonically decreasing peak stress with increasing temperature from about 50 MPa at -40ºC to about 10 MPa at +80ºC. The fracture energy is shown to be relatively insensitive to the variation in temperature. Strain rate experiments are performed in peel mode using the double cantilever beam specimen and in shear mode, using the end notch flexure specimen. The strain rates vary; for peel loading from about 10 -4 s -1 to 10 s -1 and for shear loading from 10 -3 s -1 to 1 s -1 . In the peel mode, the fracture energy increases slightly with increasing strain rate; in shear mode, the fracture energy decreases. The peak stresses in the peel and shear mode both increase with increasing strain rate. In peel mode, only minor effects of plasticity are expected while in shear mode, the adhesive experiences large dissipation through plasticity. Rate dependent plasticity, may explain the differences in influence of strain rate on fracture energy between the peel mode and the shear mode. Keywords: Cohesive laws, Strain rate, Temperature dependence, Experimental, DCB- specimen, ENF-specimen, Crashworthiness. 1. Introduction Important design goals for modern cars are to reduce the emissions and to increase the crashworthiness. On account of this, the use of structural adhesives has become more frequent and accordingly the mechanical behaviour of adhesive joints is important. Adhesives in the form of thin layers between stiffer adherends behave differently from the adhesive as a bulk material. This is due to the constrained state of deformation. Thus, the fracture properties will depend on geometrical aspects like the thickness of the adhesive layer; cf. e.g. Kinloch (1987). It can be shown that the deformation of an adhesive layer is dominated by two deformation modes, namely peel and shear, cf. Fig. 1. Schmidt (2007) shows in an asymptotic analysis that these deformation modes dominate the behaviour of an elastic structure in small deformation provided the layer thickness is much smaller than the thickness of the joined parts, i.e. the adherends; the elastic modulus of the adhesive is much smaller than the modulus of the adherends; and the in-plane length of the adhesive joint is much larger than the thickness of the adherends. A model of the adhesive based on these deformation modes is known as an adhesive layer theory. Experiences from simulations and experiments with in-
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